1. Field of the Invention
The present invention relates to a repeater for connecting individual networks which together form a larger photonics network utilizing optical wavelength division multiplexing (WDM). More particularly, the present invention relates to a repeater which transmits an optical signal from a first network to a second network, and converts a bit rate and wavelength of the optical signal to that suitable for the second network.
2. Description of the Related Art
Optical wavelength division multiplexing (WDM) transmission systems are now under development to dramatically increase transmission capacity.
Moreover, it is intended to form an extensive photonics network by connecting different WDM transmission systems with each other. To achieve such a photonics network, various companies are developing optical add-drop multiplexers (OADM) for directly extracting and inserting an optical signal into an optical network, and optical cross-connects (OXC)for changing the path of an optical signal. For example, to provide a relatively reliable network configuration, an OADM or OXC could be used as a node in an extensive ring network.
In current WDM transmission systems in commercial use or that which will be put into practical use in near future, thirty-two (32) channels are multiplexed together, with 100 GHz signal channel space between the signals. Research and development will continue to increase the number of channels multiplexed together by narrowing the signal channel space and expanding the WDM signal wavelength band, to thereby greatly increase transmission capacity. For example, in future commercial systems, sixty-four (64) or one-hundred-twenty-eight (128) channels may be multiplexed together, and signal channel space may be narrowed to 50 GHz or 25 GHz.
FIG. 1 is a diagram illustrating a conventional optical network. Referring now to FIG. 1, optical add-drop multiplexers (OADM) 1-1, 1-2, 1-3 and 1-4 form nodes along an optical transmission path consisting of an optical fiber. Optical signals in the optical network are wavelength division multiplexed together.
FIG. 2 is a diagram illustrating a wavelength multiplexing process performed by each node 1-1, 1-2, 1-3 and 1-4, where an optical signal at a specific wavelength is branched from the transmission path, and a different optical signal at the specific wavelength is inserted into the transmission path.
Referring now to FIG. 2,a variable wavelength filter 11, such as an acusto-optical tunable filter (AOTF), is used to insert an optical signal of a particular wavelength into the transmission path. A variable wavelength filter 12, such as an AOTF, is used by each node 1-1, 1-2, 1-3 and 1-4 to branch the particular wavelength from the transmission path.
The signal wavelengths and bit rate of the optical network in FIGS. 1 and 2 might be different from that of other optical networks. Therefore, there may be difficulties when connecting such optical networks having different signal wavelengths and bit rates together.
Currently, the internationally standardized WDM signal wavelength is established at 1.55 μm band as the center frequency band with a channel space of 0.8 nm from 1535.8 nm, and at 195.2 THz in terms of the carrier frequency with a channel space of 100 GHz.
Currently, optical signal velocity (bit rate) is established at 2.5 Gb/s or 10 Gb/s, and manufacturers continue to develop an apparatus which can be connected to such signal velocities.
It is widely known that about three times the signal bit rate is enough for the signal frequency width. Therefore, when the bit rate is 10 Gb/s, the signal frequency width becomes about ±30 GHz.
In current systems, since the channel space is 100 GHz, the problem of crosstalk between adjacent channels will never be generated.
However, as indicated above, transmission capacity will be increased in future systems by increasing the number of channels (for example, to sixty-four (64) or one-hundred-twenty-eight (128)), narrowing the signal channel space(for example, to 50 GHz or 25 GHz) and expanding the WDM signal wavelength band. When the signal channel space is narrowed, a certain restriction will be given to the signal velocity (bit rate) from the point of view of signal frequency width.
If a special band compression technique is not introduced, the minimum signal channel space is assumed to become 100 GHz, for example, for 10 Gb/s and 25 Gb/s for 2.5 Gb/s.
TABLE 1bit rate (b/s)600M2.5G10G40Gnotebandwidth±1.8GH±7.5GHz±30GHz120GHzbit ratex3minimum6.25Hz25GHz100GHz400GHzchan. space(0.05nm)(0.2nm)(0.8nm)(3.2nm)(wavelengthspace)Bit rate and wavelength when 1/2n of the ITU grid 100 GHz channel space is applied.
Under this technical background, it is assumed that optical networks with different signal bit rates and signal channel spaces will be proposed. For example, one optical network might have characteristics of 10 Gb/s, 32 channels, 100 GHz channel space, while a different optical network might have characteristics of 2.5 Gb/s, 128 channels, 25 GHz channel space.
With optical networks having such different characteristics, the following problems may be assumed.
(1) Limitation on Signal Channel Space of Signal Frequency Width
For example, when a signal of 10 Gb/s is directly connected to the optical network of 25 GHz channel space, a problem is generated in which the crosstalk to the adjacent channel is generated by the signal frequency width of the signal of 10 Gb/s.
(2) Mismatch of Signal Frequency (Wavelength)
For example, when the optical network of 25 GHz channel space is connected to the optical network of 100 GHz, the signal might be changed to a signal which cannot be identified.
(3) Mismatch of Signal Velocity (Bit Rate)
When connecting optical networks of different signal bit rate, there may be a mismatch in the signal velocity of the different networks.